When a denture is worn for a long period of time, the fit between the denture and the alveolar ridge worsens. The shape of the gums changes with time as the gums recede. Since the denture is made from a hard material, it is incapable of adjusting to the change in shape of the gums. Thus, gaps begin to form between the denture and the gums. This can lead not only to the loosening of the denture, but also to an ulcer and/or inflammation due to uneven pressure exerted by the denture.
To alleviate these problems, it is necessary to provide a liner between the denture and the gum, which can both fill in the gaps that form between the denture and the gum and prevent damage to the gum. This liner may be flexible and plastically deformable, and may be capable of being used for a period of, for example, one week to a few weeks.
Numerous denture liners have been proposed to provide both comfort and better fit for the denture. Many of such liners achieve this goal by being soft, allowing them to conform to the shape of both the gums and the denture.
Conventionally, a soft denture liner is made of a synthetic resin in the form of powder, paste or a soft plate. Each time a user needs to apply a denture to the gum, the user takes the required amount of a denture base stabilizing material from a receptacle in the case of a powder or paste, or shapes it in an appropriate size and configuration in the case of a soft sheet. Conventional soft denture liners include various polymers, such as polyethylene, polypropylene, polybutene, polyvinyl chloride, ethylene-vinylacetate copolymer, nylon, polyvinyl fluoride, Teflon, polyacrylonitrile and polyvinyl alcohol.
Several soft denture liners have been disclosed, which are based on plasticized polyethyl methacrylate (PEMA). One such denture liner is disclosed in U.S. Pat. No. 5,061,182 to Kubo et al. (herein incorporated by reference). The patent discloses a denture base stabilizing sheet having a trapezoidal configuration, which is manufactured using a specific composition of PEMA, butyl phthalyl butyl glycolate (BPBG), and triacetin.
Soft liners mainly consisting of polymers of higher methacrylates, e.g., polyethylmethacrylate, a solvent, e.g., ethyl alcohol, and a plasticizer, e.g. butylphthalyl butylglycolate have also been described. These and others are disclosed in Graham et al, J., Prosthetic Dentistry, Volume 2, No. 4, pp. 422-8 (1989). The solvent and plasticizer are mixed with the polymer immediately prior to use, and the mixture is then applied to the surface of the denture. Swelling of the polymer by the solvent, and diffusion of the plasticizer into the polymer matrix cause the polymer to become soft and resilient.
U.S. Pat. No. 5,075,107 to Katakura (herein incorporated by reference) discloses a denture liner, which specifically excludes ethyl alcohol. This liner comprises a powdery component consisting of (a) either one of a copolymer of butyl methacrylate with ethyl methacrylate and a mixture of poly butyl methacrylate (PBMA) with PEMA and (b) a liquid component consisting of at least one of BPBG without ethanol, dibutyl phthalate (DBP), benzyl butyl phthalate, benzyl benzoate, ethyl benzoate, butyl benzoate and amyl benzoate, with the powdery and liquid components being mixed or kneaded together for use.
Another denture liner based on plasticized PEMA is disclosed in U.S. Pat. No. 5,436,283 to Okada et al. (herein incorporated by reference). The patent discloses a denture base lining material comprising a phthalic ester-based plasticizer containing ethyl alcohol and a powder component consisting of PEMA with or without a specific acrylate copolymer.
U.S. Pat. No. 5,513,988 relates to a known elastomeric methyl methacrylate-free soft material used as a denture reline material. The material includes powder and liquid components mixed together and cured, then used in denture applications. Where the invention involves lining over a void in an acrylic denture, the resultant mixture is applied over the underlying surface of the acrylic over the void to chemically and mechanically bond the mixture in a seamless bond to the underlying surface of acrylic. A known sealer component is then applied over the liner to create a non-absorbent exterior surface seal and glaze.
While the above conventional denture liners can provide some of the much needed relief for denture wearers, these liners have a number of problems. Conventional denture liners include polymeric materials, such as PEMA, which are chemically similar to the materials from which dentures are formed. These types of material allow the liner to be flexible and to be able to adjust to the changing shape of the gums. However, the liners tend to chemically bond to the denture, especially when the plasticizers are liberated over time. Thus, the liners become extremely difficult to separate from the denture. This is especially true with the materials discussed in U.S. Pat. No. 5,513,988 which promote chemical and mechanical bonding between the liner and the denture. Conversely, the instant invention promotes formation of adhesive bonding by providing a denture lining material which has a low glass transition temperature and can be characterized as resilient, as well as sticky or tacky.
Also, conventional denture liners do not have mucoadhesive properties. Therefore, they rely on either physically fitting the denture to the gum or on an additional adhesive. Such an approach, however, is deficient in that it leads to an inadequate hold or requires addition of a separate layer or layers of adhesives, which may interfere with the function of the liner and result in discomfort for the user.
The above-mentioned U.S. Pat. No. 5,436,283 attempts to solve the problem associated with separating the liner from the denture by including liquid paraffin and/or squalane in the composition of the liner. While such a modification helps to peel the liner from the denture, liquid paraffin and squalane also decrease the adhesion of the liner to the gum, which can lead to denture slippage.
Also problematic is the inclusion of phthalates in the plasticizer component of state-of-the art denture liners. Various advocacy groups are complaining that animal studies suggest that phthalates cause damage to an animal's internal organs such as kidneys, liver, lungs, and the reproductive system, including the testes. Since dentures and denture liners are worn in the mouth of the user where ingestion of phthalates may occur, it is desirable to produce a composition which is phthalate-free.
Accordingly, there is a need for a soft denture liner, which can provide sufficient comfort for the user and improve adhesion to the gums, while easing the process for separating the liner from the denture at the time the liner is replaced. Moreover, there is a need for a phthalate-free denture liner. Such needs are obtainable by providing denture liner compositions having a glass transition temperature lower than the lower of an ambient temperature at which the liner is applied and a temperature inside the oral cavity as described below.